6

Strategic Knowledge and Observation Gaps

The “Strategic Knowledge and Observations” session considered how the physical understanding of space weather phenomena enters into the applications domain in diverse and complex ways that do not naturally lead to a simple one-to-one mapping. The goal of the basic research is to understand the causes and development of a particular natural phenomenon, be it a solar flare that produces a radio burst, a change in the magnetic field producing enhanced ionospheric current systems, or an ionospheric electrojet that produces scintillation-making structures.

While the space environment as a whole is often separated into spatial domains where different expertise is required, it is also a highly coupled system and its various behaviors are generally inter-related. For example, a user is interested in forecasts of induced currents during a geomagnetic storm. A geomagnetic storm’s intensity and progression is determined by the details of the process that creates a coronal mass ejection (CME) at the Sun that produces a disturbance with particular attributes in the solar wind at the location of Earth that in turn responds in different ways depending on conditions in the magnetosphere and upper atmosphere at the time of impact. Such chains are common and do not typically lend themselves to concise cause-and-effect relationships. However, there are usually certain missing or inadequately covered areas of knowledge that have the potential to make major differences in forecasting. To incorporate this point, the organizing committee selected a few high profile, known observation and knowledge “gaps” that, if filled, would greatly improve the prospects for achieving National Space Weather Action Plan (NSWAP) forecasting goals.

SUN/HELIOSPHERE GAPS

The Sun is the ultimate driver of much of Earth’s space weather environment, through its effects on Earth’s surface and atmosphere at its lower boundary, to the variable solar wind particles and fields that both form the magnetosphere and control the planets’ exposure to galactic and solar cosmic rays (see Figure 6.1).

The strategic knowledge that is central to the nowcasting and forecasting of solar effects concerns the state of the solar magnetic fields, the coronal structure that depends on those fields, and the related electromagnetic emissions in a broad range of wavelengths from radio to X ray. Various users of this information are often interested in (1) the general fluxes of solar extreme ultraviolet (EUV) emission over time scales of days to years—for use in various space weather models ranging from satellite drag/orbit decay to ionospheric properties, (2) the level of solar activity in the forms of flares and CMEs that are precursors to disruptions of systems such as power grids and pipelines, Global Positioning System (GPS) and other spacecraft, and space launch operations; or (3) the fluxes of cosmic rays at commercial aviation, the International Space Station (ISS), and satellite altitudes.

The corresponding knowledge of the solar wind plasmas and fields, especially at the L1 point ~200 Re (1.5 million km) upstream of Earth where SWFO-L1 will be stationed, provides the user with the local interplanetary conditions that will affect Earth both almost instantly (X rays and energetic particles), and in roughly 30-45 minutes in cases where the incident solar wind plasma and field conditions are of interest. These conditions are often used in models—some of which are L1 data-driven—for a wide range of geospace consequences ranging from energetic particle exposure of Earth-orbiting spacecraft and the ISS, to forecasting the expected strengths of the magnetospheric ring current and auroral electrojets, and their related upper atmosphere and ionosphere perturbations.

The topics selected for this session represent long-identified, high-profile issues in improving capabilities within the National Oceanic and Atmospheric Administration’s (NOAA’s) Space Weather Prediction Center (SWPC), and within other agencies and entities that both generate and use space weather information. The first presentation highlighted the now-appreciated importance of understanding the global Sun in making both nowcasts and longer-lead forecasts for Earth’s space environment. The second presentation topic, “Bz,” or the north-south component of the local interplanetary field, was chosen in recognition of its critical importance in determining the geospace response to solar activity, and because it has proven to be a particularly difficult task to predict it for the purpose of forecasting geomagnetic storm

severity. As noted in the discussions that followed the presentation by Pete Riley of Predictive Science, Inc., an enhanced Bz at L1 from a CME that is pointing northward or southward can determine the difference between a modest geomagnetic disturbance and a Carrington-class event.¹

The third session topics address the recent trend toward realizing the potential superiority of off-Sun-Earth-axis observations for forecasting purposes. For example, the direction and velocity of an Earth-bound CME, which impacts its “geoeffectiveness,” is better determined from observations at L5 or L4 than L1. The final topic is more climatological in nature, relevant to both decadal-scale solar activity–level forecasting and is also relevant to tracking longer term trends in cosmic ray and solar EUV fluxes.

The following section is a summary and synthesis of the views expressed by the following workshop presenters:²

• Janet Luhmann, Space Sciences Laboratory, University of California, Berkeley, Session Chair
• Todd Hoeksema, Stanford University, “Value of Solar Farside and Polar Perspectives”
• Pete Riley, Predictive Sciences, Inc., “Bz Forecasting”
• Mark Gibbs, UK Met Office, “L5 Perspective”
• Natchimithuk Gopalswamy, NASA GSFC, “L4 Perspective”
• Lisa Upton, Space Systems Research Corporation, “Solar Activity Cycle Forecasting”

A theme expressed in this session was that the space weather community is now in a position to take advantage of global, whole-Sun and coronal “situational awareness,” derived largely from multiwavelength, multi-perspective imaging of the Sun combined with integrative modeling. The Sun is a constantly evolving, sometimes highly dynamic star that affects Earth’s space weather in a variety of ways, requiring different information for both long-term-trend monitoring and shorter-term nowcasting and forecasting. For example, flares—related to ionospheric perturbations and communications disruptions—are often detected and ranked by their radio and disk-integrated X-ray emissions, with EUV images providing details of their location and coronal context.

CMEs are observed in combinations of coronal EUV emissions and scattered white light beyond the limb imaged by coronagraphs. CMEs that have the largest impacts on Earth’s magnetosphere and geospace most often originate just west (to the right in the solar disk definition) of the central meridian. These typically take 2-4 days to arrive, although in extreme cases <1 day delays have been experienced. The enhanced solar wind plasma and magnetic field associated with the arrival if the interplanetary mass ejection (ICME; defined as the in situ counterpart of the CME observed in coronagraph and EUV images), can last hours or days depending on the details of the disturbances. Of primary interest is its speed, including whether it is preceded by a shock, with its following dynamic pressure enhancement from the plowed-up solar wind ahead of the coronal ejecta, and the magnetic field strength and orientation throughout its passage.

Solar energetic particles (SEPs), also referred to as solar cosmic rays, generally precede and accompany ICMEs, with their fluxes sometimes the highest upon arrival of the ICME shock. However, the most energetic (“hardest”) and fastest arriving SEPs often arise from events close to, and sometimes behind, the western limb. Considering both these possibilities, forecasting a potentially significant SEP event ultimately involves identifying potentially eruptive sites both on and behind the solar disk visible from L1. However ICMEs are particularly important to space weather watchers because they can have a broad range of space environment, upper atmospheric, and surface consequences. These may include radiation belt and

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¹ The solar flare on September 1, 1859, and its associated geomagnetic storm remain the standard for an extreme solar-terrestrial event. For a popular description of the event, see C. Klein, 2012, “A Perfect Solar Superstorm: The 1859 Carrington Event,” History.com, March 14, https://www.history.com/news/a-perfect-solarsuperstorm-the-1859-carrington-event. Technical details are at E.W. Cliver and W.F. Dietrich, 2013, “The 1859 Space Weather Event Revisited: Limits of Extreme Activity,” Journal of Space Weather and Space Climate 3: A31, https://doi.org/10.1051/swsc/2013053.

² Links to the presentations can be found at https://www.nationalacademies.org/spacewx-phaseI-presentations.

auroral enhancements, upper atmosphere heating and ionospheric current generation, and induced currents in conductors on the ground related to ring current injections associated with the dynamically changing boundary conditions presented by the ICME.

For nonexplosive phenomena, lower cadence observing is often sufficient. Coronal holes near the central meridian can produce high-speed solar wind streams and interstream compressions that modulate geomagnetic activity and the radiation belts on a regular basis-sometimes over a sequence of ~27 day solar rotations. Unlike the flares and CMEs produced by rapidly evolving active regions, these are related to relatively steady coronal structures that can have months-long lifetimes—making it possible to use solar observations well-east of the central meridian to forecast solar wind-related responses in geospace days to weeks later.

Results of longer-term investigations relating the 1 AU ecliptic measurements to the sunspot numbers and other solar activity measures are also of interest. The recent weak sunspot cycles have been accompanied by some on-average weaker solar wind parameters, including the interplanetary field magnitude and solar wind density. These, together with the related weaker solar EUV fluxes, have changed the average conditions in geospace, resulting in generally weaker geomagnetic storms and consequences. Solar cycle prediction studies not based on persistence, on the other hand, are built on real or partially physics-based modeled global solar behavior, usually relying on magnetic field and surface velocity measurements requiring synoptic observing programs providing regular, inter-calibrated images over long periods of time.

Based on this current knowledge, some participants advocated for a “whole heliosphere approach” (not just Sun-Earth line) with measurements/predictions on four timescales: (1) photon radiation events (seconds to minutes), (2) SEP events (minutes to days), (3) local magnetic field and solar wind plasma disturbances (days to years), and (4) space climate (years to centuries). In part based on the experience of whole-Sun observing with STEREO (Solar Terrestrial Relations Observatory), observations taken at the L5 and L4 Lagrangian points (Figure 6.2) have become a widely discussed option for continuous space-based imaging and in situ measurements.

The L3 location could potentially fulfill the desire for regular “farside” information on the solar magnetic fields and EUV structures invisible from Earth, but destined to rotate into view, allowing global model-based forecasting of the fields and structures expected to affect Earth several weeks ahead. While simpler line-of-sight solar surface magnetic field measurements at L3 may be adequate for this purpose (vector measurements are highly desired from L1), the same measurement cadence is needed as at L1 for global situational awareness.

The roles of similar measurements from the “side-looking” L4 and L5 perspectives are also of value for global perspective and modeling, however the three observing sites (L1, L4, and L5) would always leave a gap in the global picture if not carried out in combination with L3 measurements. For example, one

space-based observatory concept “SHOES” (Solar-Heliospheric Optical Environment Satellites) was described where a Sun-surrounding constellation could provide the essential imaging and in situ observations, based on what was available from various mission programs and sponsors. In some ways, the present Heliophysics Observatory³ is used in this spirit, NASA’s individual-mission-focused strategy (at least for the prime phase of operation) results in a system that depends to a large extent on the serendipity of the locations and timing of various missions relative to the solar and heliospheric events of interest.

A potentially revolutionary leap in knowledge would result from a similarly instrumented mission with a solar polar perspective. In particular, the out-of-ecliptic imaging perspective provides an opportunity to observe the full longitudinal extent of CMEs, including those that will perturb Earth’s space environment. While the STEREO mission imagers allowed some multidimensional model-aided rendering of CMEs, the lack of the out-of-ecliptic third viewpoint needed for three-dimensional (3D) constraints was limiting.

The Sun’s polar magnetic fields also figure prominently in the boundary conditions for coronal and solar wind modeling used in forecasting. Presently they are reconstructed using synoptic ground-based or space-based magnetograms, stitched together to form a map of the global solar magnetic field. Because the poles are poorly observed from the ecliptic perspective, schemes have been developed to “fill in” the fields there based on the occasional observations at times when the solar axis is tilted toward or away from Earth as the year progresses, or by “evolving” the surface field in time using physics-based simulations of the surface fields.

The Solar Orbiter mission, launched on February 9, 2020, will achieve a fairly high inclination (as much as 35° with respect to the Sun’s equator), has an instrument suite that provides both imaging and magnetograms,⁴ but it is in an elliptical orbit that constantly changes its perspective. Thus, while it will provide the first full observations of one pole for a period of several months every orbit, it will not provide the kind of regular synoptic information needed for space weather purposes. Nevertheless, the new insights from the Solar Orbiter may well have significant impact on the design and physics of global coronal and solar wind forecast models. The stationing of a permanent solar polar monitoring platform is desired for both research and operational purposes.⁵ Concepts relevant to achieving this future objective were presented during the workshop poster session.⁶

There are perhaps more obvious, and immediately useful observational benefits from an L5 mission, where a spacecraft is stationed at the Lagrangian position trailing Earth (see Figure 6.2). As the Sun rotates in a right-handed sense with a period of roughly 27 days, an active region or a coronal hole observed in the center of the disk by an imager at the L5 location would be in the center of the disk seen from Earth about 4 days later. The L5 perspective thus regularly provides a preview of conditions that will affect its space environment several days in advance, as long as the solar and coronal conditions evolve in a predictable

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³ NASA’s Heliophysics flight missions form a fleet of solar, heliospheric, geospace, and planetary spacecraft that operate simultaneously to understand the dynamics of the solar system. This fleet can be thought of as a single observatory, the “Heliophysics System Observatory (HSO).” From NASA, “Heliophysics System Observatory (HSO),” at https://www.nasa.gov/content/goddard/heliophysics-system-observatory-hso.

⁴ European Space Agency, 2020, “Solar Orbiter Instruments,” https://www.esa.int/ESA_Multimedia/Images/2020/01/Solar_Orbiter_Instruments. See also S.K. Solanki, J.C. del Toro Iniesta, J. Woch, A. Gandorfer, J. Hirzberger, A. Alvarez-Herrero, T. Appourchaux, et al., 2019, “The Polarimetric and Helioseismic Imager on Solar Orbiter,” Astronomy and Astrophysics 642: A11, https://doi.org/10.1051/0004-6361/201935325.

⁵ S.E. Gibson, A. Vourlidas, D.M Hassler, L.A. Rachmeler, M.J. Thompson, J. Newmark, M. Velli, A. Title, and S.W. McIntosh, 2018, “Solar Physics from Unconventional Viewpoints,” Frontiers in Astronomy and Space Sciences 5, https://doi.org/10.3389/fspas.2018.00032.

⁶ These included HISM (The High Inclination Solar Mission) and Solaris (SOLAR sail Investigation of the Sun). HISM is a concept study, conducted at the Marshall Space Flight Center, Advanced Concepts Office (ACO), which would use a large solar sail for thrust to maneuver out of the ecliptic. Solaris, led by Don Hassler of the Southwest Research Institute, is one of five science investigations selected as a possible future NASA MIDEX mission. It would first travel to Jupiter and use the planet’s gravity to slingshot out of the ecliptic plane and fly over the Sun’s poles at 75 degrees.

(modelable) way. In addition, when an Earth-directed CME occurs, the L5 imagers obtain details of its direction, latitudinal extent and speed that cannot be readily obtained from Earth’s perspective.

In situ instruments would measure the solar wind plasma and field properties that would be experienced at L1 in several days provided that the coronal structures controlling the ecliptic solar wind streams maintain their configuration over that time span—a situation often realized during undisturbed times. The desirable instruments for an L5 mission are mostly the same as those for L1 missions, including EUV imagers and coronagraphs, plus in situ instruments (solar wind plasma, magnetic field, and energetic particles). However, participants noted that a magnetograph is also important for the purpose of evaluating the evolution of magnetic fields rotating onto the Earth-facing disk, including flare and CME-producing features.

Similarly, a Heliospheric Imager, like those on the STEREO mission to image the propagation of CMEs from the corona to Earth in faint scattered white light, provides the opportunity to monitor the evolution of the structure as it interacts with the ambient solar wind along the way—affecting its properties and time of arrival. Considering there is currently +/−12 hours accuracy in the estimated arrival time for an ICME based on current (L1) observations alone, the regular availability of a heliospheric imager watching the Sun-to-Earth space would represent a significant advancement in predictive space weather capability. Plans in process for an European Space Agency (ESA) L5 mission, “Lagrange,” are perhaps the most developed at this writing. Both NOAA and NASA are involved in that endeavor in various collaborative roles.

There are also distinct advantages of an alternative (or added) L4 Lagrangian point monitor, located at the same longitudinal separation, but instead leading Earth in its orbit (Figure 6.2). This location would allow for the same Sun-to-Earth imaging of Earth-directed CMEs as the L5 point. Moreover, this additional perspective on CME/ICME appearance and behavior would further constrain any predictions of Earth impacts, give the added ability to properly visualize, orient and infer the direction of the 3D, traveling structure. Although an L4 monitor would not have the ability to preview solar features and solar wind structures that would affect Earth days later, it would replace that L5 contribution with a different unique ability for major SEP event forecasting.

CMEs/ICMEs move predominantly in the radial direction from the Sun, extrapolation of their average trajectory is fairly straightforward-even though they change shape and speed. However, SEPs to first order travel from their sources in flares or at interplanetary shocks along the Archimedean spiral magnetic field lines of the Parker Spiral interplanetary field.⁷ The most energetic SEPs are generally born close to the Sun in either flare sites or at fast CME shocks when they are still low in the corona. Tracing the nominal Parker Spiral shape from Earth back to the Sun intersects the Sun west of the central meridian, where ~80% of SEP events originate. Sometimes this location can be near or behind the west (right hand in images) limb, making the source event/region invisible from L1 (and especially from L5).

Occasionally SEPs can reach ~GeV energies and travel at nearly the speed of light along the Parker Spiral field to Earth, at about the same time the flare or fast CME is detected in solar images. Observing the magnetic field structure of evolving active regions from L4 could contribute to early pre-event warnings or could contribute to “all clear” forecasts when eruptions are deemed unlikely. An EUV imager at L4, supplemented by an X-ray detector and radio antenna for detecting the Type II radio bursts associated with a shock wave in the corona, would provide the possibility for several minutes warning for astronauts on an EVA in an exposed part of near-Earth space.

An added benefit of the L4 location for Earth-directed CME/ICME imaging is that this location avoids the SEP-produced “snowstorm” of background noise in the solar images because of its geometry relative to Earth. It was suggested that the combination of L1, L5, and L4 missions with similar instrumentation

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⁷ “The heliospheric magnetic field (HMF) is the extension of the coronal magnetic field carried out into the solar system by the solar wind. It is the means by which the Sun interacts with planetary magnetospheres and channels charged particles propagating through the heliosphere. As the HMF remains rooted at the solar photosphere as the Sun rotates, the large-scale HMF traces out an Archimedean spiral.” From M.J. Owens, R.J. Forsyth, 2013, “The Heliospheric Magnetic Field,” Living Reviews in Solar Physics 10: 5, https://doi.org/10.12942/lrsp-2013-5.

would fulfill most needs for a space weather infrastructure with improved outcomes based on observables alone.⁸

Attention was also focused at the workshop on a particularly important and challenging forecasting problem without which Earth impacts are essentially impossible to evaluate: inferring the direction and strength of the out-of-ecliptic (North-South) component of the interplanetary magnetic field Bz in Earthbound ICMEs. As mentioned above, because of the nature of the solar wind interaction with Earth’s nearly dipolar global field, a large southward component of the interplanetary field literally opens the magnetospheric shield external driving and other influences. Thus an ICME which carries large southward Bz can cause major consequences compared to the same ICME with a flipped Bz polarity.

Research has led to various Bz forecast attempts and techniques: mechanistic, statistical, and empirical. Most of the long lead-time approaches have used solar magnetograms and coronagraph images to envision the magnetic topology of the erupting 3D structure. The structure is then either assumed to simply expand outward to 1 AU, with their basic geometry unaltered by the solar wind interaction, or is fed into a solar wind propagation model/simulation that provides at least a rough estimation of the expected sign of the arriving Bz. But there are intrinsic uncertainties in this methodology: 45 percent of ICMEs have no observed solar event, including 10 percent producing moderate-to major geomagnetic storms. Only 56 percent of halo CMEs produce the flux rope-like magnetic clouds at L1 often assumed by these models.

The other observational possibilities are either remote-sensing the sign of Bz in the CME/ICME views from the side (e.g., L5 or L4) using radio signal Faraday rotation, light polarization along the Sun-Earth axis, or one or more spacecraft measuring in situ plasma and field closer to the Sun than L1. Numerical simulations of the full eruption and propagation process provide an alternative, but these must be data-driven and perhaps assimilative to address real events, and the results of such efforts to date suggest they are not yet in a state to be used operationally. More empirical projections can be made based on previous statistics and probabilities; for example, the ICME flux ropes show systematic solar cycle trends in observed Bz behavior, but these are best suited to climatological uses rather than to individual events. Forecasting the interplanetary field orientation out of the ecliptic, especially in Earth-directed ICMEs, remains a top priority and goal of space weather forecasting for the coming decade.

Long-term solar activity forecasts are of value for advance planning of operational monitoring projects as well as for anticipating extremes in space weather conditions. Every solar cycle NOAA and NASA convene a panel to consider the range of sunspot number predictions offered by the broader scientific community and various space weather forecasting centers, in order to discuss their merits and release an “official” value of the maximum and length. There are 61 sunspot number predictions for Solar Cycle 25. Cycle 24, which lasted 11.4 years, is the fourth smallest on record and the smallest for the Space Age.

An analysis released in 2019 forecasts Cycle 25 to be of comparable magnitude and length to Cycle 24. Part of the rationale behind this latest prediction is that the active region gap that preceded Cycle 24 appears to be similar for Cycle 25. Strong cycles start with active regions at higher latitudes and have significant overlap in their temporal distributions. Both 24 and 25 started/are starting at lower latitudes and exhibit extended sunspot minima including the gap (large number of spotless days) between the cycles in the standard time series and butterfly diagrams.

Of the 61 predictions considered by the panel, significant fractions used numerical methods (e.g., statistics, neural networks), while others used physics-based methods (e.g., polar fields or other precursors, surface flux transport, dynamo modeling). The polar field precursor method is a particularly popular approach, which is of interest considering the lack of observations and understanding of the polar fields described above. The polar fields also figure in the physics-based models, such as surface flux transport, which relates the polar field behavior to the decaying active region magnetic fields of the previous cycle—providing a “memory” or seeding effect from cycle to cycle. The latter is potentially powerful for long-term predictions of solar activity although the physical details of the surface field connections to the solar

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⁸ Several other mission concepts, presented in the workshop poster session, would expand solar longitude coverage. They include Magnetic Explorer, a Pearl Necklace Concept, and Solar Cruiser.

dynamo activity remains to be understood. Only recently has the science of helioseismology had a long enough and consistent enough record from which to study the solar cycle development in the interior.

On the basis of the solar/heliosphere session presentations, which necessarily focused on only a few of the highest profile topics of interest toward defining future space weather infrastructure, it is possible to see several specific directions of major potential value for the post SWFO-L1 era. Prominent among these was the view that multi-perspective imaging and distributed/multipoint in situ measurements were highly desired. Global perspectives on both the Sun and corona, as well as in the inner heliosphere provide what was deemed invaluable situational awareness and fodder for assimilative model forecasting that can be realized in the next decades. These can address some of the NSW-SAP initiative outstanding goals such as longer-lead forecasting for both geomagnetic storms and space radiation hazards.

For predictions of a major space weather event, the capability to watch a large and complex active region evolve as it rotates onto the visible disk, especially in its magnetic field configuration and EUV emission features, is highly desired. Special challenges such as the prediction of Bz will require modeling/simulation developments that are discussed in a later session of this workshop proceedings. Similarly, solar cycle predictions will require both model development and longer timelines of helioseismology observations to fulfill their promise. Simply realizing action on the set of items featured in this workshop would represent the kind of major advancement sought in the SWFO program of the future.

Table 6.1 provides a summary of workshop presentations and discussions regarding solar and heliospheric knowledge gaps, and options to those fill gaps.

GEOSPACE GAPS

The “Strategic Knowledge and Observations—Geospace” session was organized in a manner similar to the “Sun/Heliopshere Gaps” session summarized above. Speakers at this session were the following:⁹

• Mary Hudson, National Center for Atmospheric Research (NCAR)/High-Altitude Observatory (HAO) and Dartmouth College, Session Chair
• Drew Turner, Johns Hopkins University Applied Physics Laboratory (JHU/APL), “Inner Magnetosphere Measurements”
• Delores Knipp, University of Colorado, Boulder, “Low Altitude Measurements—Particles”
• Brian Anderson, JHU/APL, “Low Altitude Measurements—Fields”
• Tim Fuller-Rowell, University of Colorado, Boulder, and NOAA, “Ionosphere Measurements”
• Eric Sutton, University of Colorado, Boulder, “Satellite Drag”

The session covered inner magnetosphere measurement requirements (Turner), particles (Knipp) and fields (Anderson) measurements at low altitudes, and ionosphere (Fuller-Rowell) and thermosphere (Sutton) measurement gaps. Figure 6.3 shows a sketch of an interplanetary shock produced by a CME from the Sun impacting Earth’s magnetosphere driving modifications to the radiation belts and enhanced current systems and particle precipitation at high latitudes that ensue from this type of space weather event. Figure 6.4 shows a complementary sketch of the ionosphere-thermosphere system which is coupled to solar and magnetospheric input as well as drivers from below. Dynamic processes in this complex coupled system drive space weather effects extending throughout the domain shown in both figures.

Inner magnetosphere space weather threats related to charged particle radiation effects that require ongoing measurement include the following: (1) total ionizing dose due to MeV electrons and multi-MeV protons, which cause damage to spacecraft over hours to years timescales; (2) single-event effects due to spacecraft-penetrating multi-MeV particles, which can cause instantaneous disruption of spacecraft performance; (3) internal charging due to >100 keV electrons; and (4) surface charging due to ~keV

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⁹ Links to the presentations can be found at https://www.nationalacademies.org/spacewx-phaseI-presentations.

TABLE 6.1 Solar/Heliosphere Knowledge Gaps, Space Weather Operations Impacts, and Measurement Options

NOTE: This table is a synthesis derived from session presentations and discussions.

electrons. Additional space weather threats for the ionosphere/thermosphere/ground and orbital threats at low Earth orbit (LEO) are discussed below. In discussions, it was observed that additional triage and resolution analysis (forensics) as well as spacecraft design require measurements over disparate timescales, distinguishing between space weather for spaceflight operational decisions and space climatology for space mission design considerations.

A concern of some participants is how radiation belt and ring current energy populations will be monitored going forward now that the NASA Van Allen Probes mission (2012–2019) has ended, as will measurements of energetic electrons and protons at LEO by the National Oceanic and Atmospheric

Administration (NOAA) Polar-orbiting Environmental Satellite (POES) and Department of Defense (DoD) DMSP (Defense Meteorological Satellite Program) spacecraft. In addition, low-energy plasmaspheric electrons, ~eV energy, play a critical role in determining the inner edge of outer zone radiation belt electrons and an important boundary for spacecraft surface charging. Concerning internal and surface charging threats, it was noted that spacecraft charging/discharge monitors on every spacecraft could provide a low-cost resource for anomaly resolution and model development short of direct measurements of the energetic particle environment.

There are differences between the information that users want or need and what scientists study in order to understand the system variability. Operators desire accurate (preferably data assimilative) nowcasts/forecasts of environmental quantities related to various hazards and user-friendly anomaly diagnostic tools. Designers could use climatological models for specific spacecraft and locations. Versions of a Space Environment Monitor on board DMSP and NOAA POES spacecraft began measuring fluxes

and boundaries of low-to-medium energy particles in the late 1970s. The relatively simple measurements have produced an important climatology of boundaries and particle effects for strong and weak solar cycles.

Commercial data buys may provide such energetic particle measurements in the future and will be most useful for improving forecast models if made publicly available by NOAA. Measurements of the geomagnetic cutoff translates to a boundary for SEP access to high latitudes. SEPs are a known space weather threat both to commercial aircraft via disruption of communications as well as radiation exposure threat to crew. Further, the ISS and its crew spends more time above SEP cutoff latitudes during active times when the polar cap is expanded.

The Iridium satellite constellation of 66 (75 including on-orbit spares) high-latitude spacecraft for global phone and data communications, recently replaced with the NEXT constellation (Iridium 2nd Generation Satellite Constellation), is equipped with magnetometers now used to map global magnetic field aligned current systems through the AMPERE (Active Magnetosphere Polar Electrodynamics Response Experiment) project funded by the National Science Foundation (NSF). Monitoring these current systems in real time, combined with global magneto hydro-dynamics (MHD) models of solar wind-magnetosphere--

ionosphere coupling, can facilitate ground induced current (GIC) forecasting, a significant space weather threat.

The global Birkeland current measurement provides a stringent validation check of the operational MHD models used to derive space weather predictions of LEO and ground impacts. The global, continuous nature of the Iridium sampling via AMPERE complements ground magnetometer observations as a validation check of the operational MHD models used to derive space weather predictions of LEO and ground impacts. The Birkeland currents and LEO magnetic observations also provide key inputs for thermospheric heating, and ionospheric electrodynamics specification, which informs scintillation and GIC forecasts. These observations are, in principle, available in real time and resample the entire orbit tracks every 90 minutes.

In situ measurements of energetic ion and electron flux have been provided in the energy range of 30 keV to more than 200 MeV by instruments on NOAA’s POES and similarly instrumented MetOp spacecraft, developed by the ESA and operated by EUMETSAT (European Organisation for the Exploitation of Meteorological Satellites). The last of the spacecraft in the POES series, NOAA-19, launched in 2009 while MetOp-C, the last in the MetOp series, launched in November 2018. The POES series follow-on is the Joint Polar Satellite System (JPSS); JPSS-1 successfully launched on November 18, 2017, and is now identified as NOAA-20. The second generation of MetOp, MetOp-SG, is composed of two series of spacecraft, MetOp-SG A and B, flying on the same mid-morning orbit as the current MetOp satellites. Launch of MetOP-SG A1 is planned for 2024 and MetOP SG B1 is planned for launch in 2025.¹⁰

MetOp-SG will continue measurement of auroral particles affecting ionospheric conductivity and radiation belt populations, SEPs, and the lower-energy galactic cosmic rays. Enhanced fluxes of these particles entering the atmosphere can produce significant and widespread degradation in short-wave radio propagation; in extreme cases even radio blackouts. The energetic particles also contribute to astronaut radiation exposure, especially on high-inclination-orbit missions during energetic solar particle events. However, as noted earlier in this report, energetic particle measurements are not part of the measurements planned on JPSS.

In addition to the loss of energetic particle measurements in next-generation POES, there is concern that drift-meter measurements from DMSP, which provide equivalent electric field measurements, will be lost. This measurement can be replicated on a global scale, but not locally, to understand ionospheric convection patterns using the NSF-supported SuperDARN (Super Dual Auroral Radar Network) ground-based radar network. These measurements, which determine the cross polar cap potential, are critical to validating global MHD models of the magnetosphere. It was also noted at the workshop that AMPERE and SuperDARN could provide operationally useful data streams, while SuperMAG,¹¹ a worldwide collection of geomagnetic ground station data with disparate sources, is useful for retrospective analysis of space weather events. Improved knowledge of surface conductivity remains critical to addressing the GIC problem.

The thermosphere-ionosphere system is currently modeled with 10-100 times lower data assimilation constraint than terrestrial weather, and yet it can change on a (10 times) faster timescale. Whereas 6-hour data assimilation cycling is adequate for terrestrial weather, the upper atmosphere and ionosphere requires 5- to 15-minute updates of the state due to the rapid response to geomagnetic activity. Monitoring the ionospheric plasma and its structure is needed to help mitigate the impact on communications and navigation. Total electron content (TEC) measurement using the GPS satellite constellations provides valuable data to constrain the system but is highly nonuniform in global distribution of ground stations with two-thirds of the globe covered by oceans. A system of buoys for TEC measurements combined with satellite-based radio occultation measurements to produce ionospheric density profiles for assimilation into models is needed.

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¹⁰ European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT), 2020, “Metop—Second Generation,” https://www.eumetsat.int/metop-sg.

¹¹ J.W. Gjerloev, 2012, “The SuperMAG Data Processing Technique,” Journal of Geophysical Research 117: A09213, doi:10.1029/2012JA017683.

Global coverage of thermospheric neutral density is also required for improvement in satellite drag and space traffic management in LEO, for orbit prediction and collision avoidance. It is imperative that physical modeling and data assimilation capability develop alongside improvement in observations. With a robust modeling capability, it opens the door to characterizing the upper atmosphere and ionosphere space weather by monitoring the drivers of the system. To this end, observations of plasma drift, neutral composition, neutral dynamics, and the source of energy from the magnetosphere (Poynting flux) and solar radiation (EUV flux) are required.

There have been 45,000 objects launched and tracked during the space age with over 17,000 currently in or passing through LEO. With Iridium (75), PlanetLabs (200), Spire Global (100), and now SpaceX (600) planning to launch 60 satellites every 2 weeks to configure the 12,000-satellite Starlink global internet service, there is a growing concern and need for satellite debris and satellite conjunction analysis. Forecasts that include collision avoidance depend sensitively on the neutral density profile, which in turn is strongly affected by episodic energy deposition during space weather events.

Recent studies have shown that low-to-medium–energy (0.5–20 keV) particle deposition significantly modulates electromagnetic energy transfer from the magnetosphere to the ionosphere-thermosphere by ionization changes in ionospheric conductivity. Loss of such measurements will hamper future neutral-atmosphere modeling efforts required to keep up with collision-avoidance needs in the increasingly busy LEO environment. Mass density measurements made by accelerometers on a limited number of scientific satellite missions are ongoing. There is however a need for improved chemical composition measurements to guide orbit prediction methods based on models of the neutral atmosphere.

Relevant to filling these knowledge gaps and needs are plans that include NASA’s implementation of the Geospace Dynamics Constellation;¹² NSF’s support for the SWARM-EX CubeSat mission,¹³ an initial three-satellite pathfinder towards a larger constellation of 6 to 12 CubeSats, each with a more elaborate suite of instruments; and ESA’s Daedalus mission,¹⁴ a concept that is based on a mother satellite, which carries a suite of instruments along with four small satellites carrying a subset of instruments that are released into the atmosphere. It was also noted that space traffic management, which requires improved models of the neutral atmosphere, will become a shared DoD-NOAA responsibility as space commerce increases dramatically in the current decade (Figure 6.5).

Table 6.2 summarizes workshop presentations and discussions regarding geospace knowledge gaps and options to fill those gaps.

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¹² See NASA, 2019, GDC STDT Final Report, https://science.nasa.gov/files/science-pink/s3fspublic/atoms/files/Oct_2_4b_GDC%20STDT%20Report%20to%20HPAC_Ridley_Jaynes--20191002.pdf.

¹³ J. Zehnder, 2019, “Building a Satellite Swarm to Investigate an Atmospheric Anomaly,” News and Events, Ann and H.J. Smead Aerospace Engineering Sciences, University of Colorado, Boulder, October 16, https://www.colorado.edu/aerospace/2019/10/16/building-satellite-swarm-investigate-atmospheric-anomaly.

¹⁴ See the Daedalus.earth website at https://daedalus.earth/.

TABLE 6.2 Geospace Knowledge Gaps and Measurement Implementation Concepts and Plans